The greenhouse effect. It’s the mechanism warming the world as greenhouse gas levels rise. But it’s not all bad. If Earth had no greenhouse effect at all, it would be too cold for us altogether. Just look at frigid, dry Mars.

Many thought that if Mars had a stronger greenhouse effect, it might be more hospitable to life. It’s not a crazy idea. Mars has lots of carbon dioxide—much of it frozen. In the past, the CO2 as gas could have been in the atmosphere. And previous studies suggested that a denser CO2 atmosphere could have produced a warm and wet Martian climate.

Mars has dry canyons and riverbeds, so water must have flowed at some point. But those flows may have been short-lived melting episodes triggered by volcanism or asteroid impacts. Because the idea of a balmy ancient Mars just got iced.